1. Field of the Invention
The present invention relates to a planar surface illuminator, and especially to a planar surface illuminator using point light sources and provided below a liquid crystal display (LCD) panel.
2. Description of the Related Art
Most users expect displays in portable devices, such as laptop and notebook computers, mobile phones and game devices, to have large, clear, bright viewing screens, equaling the performance of the cathode-ray-tube (CRT) monitors sitting on their desks. LCDs are one type of flat panel display (FPD) which satisfies these expectations. However because liquid crystals are not self-luminescent, LCDs need a planar surface illuminator which offers sufficient luminance (brightness) in a planar surface. Typically, planar surface illuminators have one of two types of light sources, one being linear sources, such as a cold cathode fluorescent lamp (CCFL), and the other being point sources, such as a light emitting diode (LED). Different types of light sources require different planar surface illuminator design structures.
As shown in FIG. 5, a conventional planar surface illuminator 10 which uses point light sources comprises a light guide plate 15 and three point sources 13 positioned at one side of the light guide plate 15. The light guide plate 15 couples with light beams emitted from the point sources 13 to create a planar surface illuminator for irradiating a liquid crystal panel (not shown). The point sources 13 are LEDs, each of which has an emission beam known as a Gaussian beam. The Gaussian beam has an optical intensity distribution function shaped like a Gaussian curve.
In operation, the Gaussian beams from the LEDs 13 irradiate an end surface (not labeled) of the light guide plate 15, and may transmit in the light guide plate 15 or may be emitted out of the light guide plate 15 through an output surface (not labeled), which is perpendicular to the end surface. As seen in FIG. 5, lower intensity parts of the Gaussian beams illuminate the areas D, E, F, G between and adjacent each two LEDs 13. In some areas near the mid-point between each two LEDs 13, almost no beams are emitted out of the light guide plate 15. Darkened areas are formed near points D, E, F, and G. Therefore, the planar surface illuminator 10 cannot produce uniform brightness over an entire area of the liquid crystal display panel.